Level converters are used, in a known manner, to convert a first voltage which is based on a first potential to a second voltage which has the same magnitude and is based on a second potential. Level converters are used, for example, if the first voltage, on account of the first potential on which it is based, is outside the operating range of a signal processing circuit which is intended to process the voltage further.
FIG. 1 illustrates one application of such a level converter and one example of a known level converter. This figure illustrates an inductance-exhibiting load 100, one connection of which is at a reference-ground potential GND and the other connection of which is selectively connected to a supply potential V+ or reference-ground potential GND via a half-bridge which comprises two switches SW1, SW2. Arrangements of this type are found, for example, in switching converters or in electric motors, the inductive load representing one of a plurality of motor windings in the last-mentioned case.
In order to detect a current flowing through the load, a measuring resistor (shunt resistor) Rs is connected in series with the load L, a current flowing through the load giving rise to a voltage drop Vin across said measuring resistor, which voltage drop is intended to be processed further by a measuring amplifier OPV in order to determine the current flowing through the load. This voltage Vin is based on a potential at the node which is common to the load L and the measuring resistor Rs. This potential varies on the basis of the switching state of a first and a second switch SW1 and SW2 and corresponds approximately to the supply potential V+ when the first switch SW1 is closed and the second switch SW2 is open.
Assuming that this voltage Vin is outside the operating range of the measuring amplifier OPV, it is necessary to convert this voltage to a voltage Vout which has the same magnitude and is based on a lower potential.
To this end, the known level converter comprises two identically dimensioned series circuits each having a resistor R10, R20 and a current source Iq10, Iq20, which are each connected between the terminals 110, 210, between which the first voltage Vin is applied, and reference-ground potential GND. In this case, the second voltage can be tapped off between the nodes which are respectively common to the resistor R10, R20 and the current source Iq10, Iq20 of a series circuit. In comparison with the first voltage Vin, the second voltage Vout is based on a lower potential which is below the potential at the node 110 by the value of a voltage drop V10 across the resistor R10. This voltage drop is prescribed by the resistance of the resistor R10 and the current provided by the current source Iq10 which is connected in series with said resistor R10.
The disadvantage of this level converter is that the potential on which the second voltage Vout is based fluctuates to the same extent as the potential on which the first voltage Vin is based. The so-called “common mode range” defines the range within which the potential on which the first voltage is based changes. In the case of the known level converter, this common mode range must not be greater than the operating range of the downstream amplifier OPV. The known converter operates in a comparatively inaccurate manner on account of production-dictated or temperature-dictated fluctuations in the resistances of the resistors R10, R20. This is problematic, in particular, when the common mode component of the first voltage Vin is large in comparison with the first voltage Vin, that is to say when the potential on which the first voltage Vin is based is considerably larger than the first voltage Vin itself. Conventional ratios between the common mode component and the useful component, i.e. the first voltage Vin in the example explained above, may be between 100 and 1000.
The data sheet IR2171/IR2172 (S), International Rectifier, Jan. 27, 2004, describes an integrated module which uses a measuring resistor that is connected in series with a motor winding to convert a voltage to a pulse-width-modulated signal based on a reference-ground potential. In this case, the pulse width of the signal obtained contains the information about the magnitude of the measurement voltage across the measuring resistor. However, this circuit can be implemented only with a high outlay. For these and other reasons, there is a need for the present invention.